1. Field of the Invention
The present invention generally relates to a method for forming assist feature patterns, and more particularly, to a method for forming assist feature patterns that are able to reduce reticle heating and to mitigate reticle distortion in the photolithography.
2. Description of the Prior Art
Integrated circuit (IC) is constructed by devices and interconnections, which are formed by patterned feature in different layers. In the fabrication of IC, photolithography has been an essential technique. The photolithography is to form designed patterns such as implantation patterns or layout patterns on at least a photomask, and then to precisely transfer such patterns to a photoresist layer by exposure and development steps. Subsequently, by performing semiconductor processes such as ion implantation, etching process, or deposition, the complicated and sophisticated IC structure is obtained.
During the exposure step, a photoresist layer is spinned—on a wafer, and a photomask is exposed to deep UV lights to form latent images in the photoresist layer. And the exposed portion or unexposed portions are removed during the development step subsequently to the exposure step depending on the types of the photoresist layer (positive or negative). Thus the patterns on the photomask are transferred to the photoresist layer.
As mentioned above, the photomask is exposed to deep UV lights such as 248 nanometer (hereinafter abbreviated as nm) KrF deep UV or 193 nm deep UV in the exposure step. Typically, the deep UV forms the latent images in the photoresist layer through portion of the photomask, such as portions having great light transmission rate. However, at portions having lower light transmission rate, the photomask absorbs energy from the exposing light, resulting in reticle heating. The reticle heating causes thermal expansion and an undesirable problem: reticle distortion. As a result, the reticle will increase overall in temperature and this increase may be uneven and causing unpredictable and non-correctable distortion.
It is noteworthy that since the IC critical dimension is keeping shrinking, requirement to resolution and overlay control of the photolithography are getting stricter and stricter. During fabrication, the effects of reticle heating can be evaluated as a function of heat induced overlay error that renders severely adverse impact to pattern transfer accuracy and process yield.